Hybrid engines typically include an electric motor and an internal combustion engine. Engines, including diesel engines, gasoline engines, gaseous fuel-driven engines, and other engines known in the art, traditionally exhaust a complex mixture of pollutants. These emissions may include gaseous and solid material, such as, particulate matter, nitrogen oxides (NOx), and sulfur compounds. Heightened environmental concerns have led regulatory agencies to increase the stringency of emission standards for such engines, forcing engine manufactures to develop systems to further reduce levels of engine emissions.
Various devices and methods are used to reduce emission levels, including after-treatment devices such as nitrogen oxide absorbers, sulfur oxide absorbers, and hydrocarbon catalysts. These devices operate by reacting exhaust gases with a chemical catalyst to transform pollutants into less harmful products, such as water and nitrogen. One method for reducing exhaust emissions is selective catalytic reduction (SCR). During SCR, a catalyst facilitates a reaction between ammonia and NOx to produce water and nitrogen gas, thereby reducing NOx levels in the exhaust gas.
While catalysts can reduce emission levels, they generally operate most efficiently within a limited temperature range. However, exhaust gas temperatures can fluctuate dramatically during normal engine operation, and these fluctuations can significantly reduce catalytic efficiency. Several devices and methods have been used to maintain suitable exhaust gas temperatures, including heating systems such as fuel burners, microwave technology, and electric heaters.
One method for controlling exhaust temperature is disclosed in U.S. Patent Application Publication No. 2007/0017215 (hereinafter “the '215 application”) of Matheaus et al., published on Jan. 25, 2007. The '215 application describes a hybrid engine and a lean NOx trap (LNT) after-treatment system. The system described by the '215 application varies the torque loading on the hybrid engine using an electric motor. Increased loading raises exhaust temperatures and thus reduces temperature fluctuations experienced by the LNT. Additionally, heaters powered by electrical energy supplied by the hybrid motor can be used to heat the exhaust.
Although the system of the '215 application may reduce emission levels, LNTs have several limitations. LNTs require frequent regeneration, such as, every thirty seconds. Also, the system of the '215 application requires torque sensors or other devices to dampen engine oscillations caused by frequent LNT regeneration, and frequent switching between rich and lean operating conditions can adversely affects engine efficiency.
The present disclosure is directed at overcoming one or more of the limitations in the prior art.